In a tough neighbourhood, there’s just four ways to survive…
- You can hide (wimpy, but it works).
- Like Batfink, you can be tougher than everyone else: ‘Your bullets cannot harm me. My wings are like a shield of steel’.
- If you go down fighting, we can re-build you, Terminator-style.
- Or you die. But your kids come back to avenge your death.
In flammable ecosystems, plants have exactly the same options.
- Plants can ‘hide’ – or more accurately, avoid fires – by growing in places that are rarely burnt, like rocky outcrops.
- Thick bark enables many species to ‘tough it out’ during intense fires, by insulating their delicate buds from heat.
- If their above-ground tissues are killed completely, many species can re-grow from buds below the ground.
- And, if they die, their offspring can re-claim the site, by regenerating from seeds stored in the canopy or the soil.
To remember this list for an exam, repeat the mantra: (1) Hugh Grant, (2) Batfink, (3) Arnold Schwarzenegger, (4) John Wayne.
But wait, there’s one more option. In teen movie after teen movie, the heroine overcomes harsh growing conditions by – everyone knows this one – (5) by opening a dance school. (Better add #5, Patrick Swayze to the list too).
The teen dance movie uses a completely different approach to survive in a challenging environment. The goal is not to withstand disturbances, it’s to tame them. To quench the flames of violence – through dance and romance – before you get burnt. And, believe it or not, some plants use this option too.
Another way to persist in a flammable environment is to make your environment less flammable.
Plants can reduce flammability in many ways. Some species of conifers evolved with fire and shed their lower limbs as they grow. This makes it hard for ground fires to enter the tree canopy. Patches of dense trees can inhibit the growth of plants beneath them, which reduces ground fuels. Some trees produce litter that burns poorly. Each of these mechanisms can enhance plant survival by reducing fire intensity.
Recently, we investigated whether this explanation might explain how the fire-sensitive tree, Callitris glaucophylla (White Cypress-pine), manages to persist in flammable eucalypt woodlands.
White Cypress-pine is a slow growing, long-lived, non-resprouting tree, and is usually killed by fires that burn 100% of its foliage. Seeds are held in cones, but these open each year, so trees don’t accumulate a large seed store in the canopy. Thus, White Cypress-pine is weakly serotinous compared to other Callitris species. Seeds germinate rapidly or are eaten when they drop to the ground, so it doesn’t form a soil seed bank either. This is a risky strategy for a slow-growing, fire-killed, non-resprouting tree.
Pop Quiz # 1 – which of the 5 movie survival strategies are available for White Cypress-pine? Can it mimic John Wayne or just Hugh Grant?
Despite this, White Cypress-pine is common in many flammable habitats. In some places, it can avoid fires by ‘hiding out’ among rocks. It grows on rocky ridges in central Australia, for example. But in many places in northern and eastern Australia, it grows on flat plains dominated by flammable grasses and eucalypts. How can it persist (and often prosper) in flat, flammable woodlands?
Callitris in the Pilliga
Recently, one of my PhD students, Janet Cohn, studied whether dense clumps of White Cypress-pine could reduce fire intensity, and – if they could – whether this increased their chances of surviving large wildfires.
Janet’s study site was the Pilliga forest in northern NSW, where a large wildfire burnt 97,000 ha in 2006. Most of this burnt in a single day! Janet sampled paired plots, arranged in the direction of the fire front. In the first plot, she measured the incoming fire intensity, by measuring the size of the smallest branches that survived the fire. Twig size is a good index of fire intensity, as small twigs remain after low intensity fires, but get incinerated by more intense fires. In the adjacent, second plot, she measured three things: (1) the density and basal area of Callitris and eucalypts, (2) fire intensity (by measuring twig diameters again), and (3) the number of Callitris that survived the fire. This arrangement of plots enabled us to see whether dense patches of Callitris reduced fire intensity and improved Callitris survival, compared to isolated Callitris that grew among eucalypts.
Interestingly, we found that dense Callitris stands did indeed reduce fire intensity and increase tree survival. The more Callitris there were in the second plot, the lower the fire intensity was, and the more Callitris survived. By contrast, small isolated Callitris growing amid eucalypts experienced more intense fires, and were more likely to die.
This behaviour has some fascinating consequences. If it continued for long enough, we would expect the vegetation would get more and more ‘patchy’, with Callitris forming dense clumps and eucalypts dominating open areas. Large Callitris patches might become very stable over time, and resistant to encroaching fires. Similarly, few Callitris would be able to establish in flammable eucalypt stands. Indeed, this mechanism could easily create what ecologists call ‘alternative stable states’.
Alternative stable states?
According to alternative stable states theory, a single set of environmental conditions can sometimes support more than one type of vegetation (i.e. more than one ‘alternative stable state’). For example, a particular soil type might support an open woodland dominated by eucalypts (State 1) or a dense stand of Callitris (State 2). Patches dominated by each state might be extremely stable over long time periods, and might rarely ‘switch’ back and forwards between states.
What causes this stability? Since both states share the same soils and climate, differences between states are not caused by underlying abiotic factors. Instead, they are caused by ‘feedbacks’ between the vegetation and the environment. Each vegetation state alters the environment in a way that promotes its own existence and disadvantages the other state. Flammable states (like eucalypt savannahs and woodlands) promote frequent fires, and frequent fires, in turn, promote flammable eucalypt woodlands. Less flammable states (like Callitris stands) impede fires. This, in turn, promotes fire-sensitive species, at the expense of eucalypts and other species that regenerate after fires. Thus both states can be highly resilient, or resistant to change.
A textbook example of alternative stable states is provided by patches of fire-sensitive rainforests within landscapes dominated by flammable eucalypts. David Bowman’s book, ‘Australian Rainforests: Islands of Green in a Land of Fire’, and a superb paper by Laura Warman and Angela Moles, provide fascinating insights on alternative stable states and Australian rainforests (see reference list below).
But stable states don’t last forever. All disturbances – including fires, floods and droughts – follow one simple rule. Small disturbances of low intensity occur much more often than big ‘extreme’ events. Low intensity fires are common, but huge wildfires are rare. Alternative stable states can form if a fire-retardant vegetation state can reduce the intensity of the most frequent fires. But rare, big, extreme events can completely overwhelm this defense. If this wasn’t the case, then rainforests would continually expand and never retract (assuming that climate remained stable) .
Extreme events and Callitris
So how do extreme events affect Callitris stands? Do Callitris patches reduce fire intensity and persist through huge wildfires? The Pilliga was a great region to study this question. In November 2006, the forest was burnt by a huge wildfire. In the first 24 hours, a high intensity fire burnt 70,000 ha in hot, dry and windy weather. As weather conditions improved, fire intensity reduced. Over the next two weeks, the fire burnt another 27,000 ha. This enabled Janet to compare fire patterns and Callitris survival between adjacent areas that burnt under extreme and more moderate conditions.
Janet’s results were not unexpected. Under ‘normal’ fire weather conditions, Callitris patches reduced fire intensity by a big enough margin to let many Callitris survive. The more Callitris there were in a patch, the more the fire intensity was reduced, and the more Callitris survived. But under extreme fire weather conditions, the wildfire burnt at high intensity everywhere. Dense Callitris patches did reduce fire intensity by a small amount, but the fire was still so fierce that it killed nearly all Callitris in the plots.
Thus, dense Callitris patches are capable of altering fire behaviour under ‘normal’ fire weather conditions. But intense wildfires overcome the feedback mechanisms that maintain Callitris stands and alternative stable states, and re-set the clock to zero.
Callitris can still persist after a high intensity fire. Seeds that were held in cones in the canopy of burnt trees fall to the ground, and seedlings can regeneration the following year. But young seedlings are easily killed by future fires, even those of low intensity. Regenerating Callitris stands have to grow large enough and dense enough before they can alter fuel characteristics and form fire-retardant patches once again.
In the words of Janet’s paper:
“By modifying vegetation and fuel structure, patches of fire-sensitive Callitris reduce fire intensity, and thereby reduce Callitris mortality, enhancing population persistence. However, this feedback loop is insufficient to ensure Callitris survival under extreme fire-weather conditions, when fire intensity is greater. After burning, stands remain vulnerable to future fires, until trees grow large enough to modify fuel levels and reduce stand flammability” (Cohn et al. 2011).
In the movies, the teen dance school is a clichéd example of educational and social programs that promote social cohesion and resilience. These programs alter social environments in positive ways, to enable communities to be more resilient to disruptive disturbances. Under normal circumstances they can work well.
Similarly, under ‘normal’ conditions, dense Callitris stands can alter the environment in ways that moderate disturbances, and promote Callitris survival and vegetation stability. But, like all biological defenses, this breaks down when times are really tough. Extreme disturbances overwhelm the processes that maintain stability, and re-set the clock to start again.
When London’s burning, attending dance class is a really lousy survival strategy.
Cohn, J.S., Lunt, I.D., Ross, K.A. & Bradstock, R.A. (2011). How do slow-growing, fire-sensitive conifers survive in flammable eucalypt woodlands? Journal of Vegetation Science 22(3), 425–435.
Bowman, D. M. J. S. (2000). Australian Rainforests: Islands of Green in a Land of Fire. Cambridge University Press, Cambridge.
Warman, L. & Moles A.T. (2009) Alternative stable states in Australia’s Wet Tropics: a theoretical framework for the field data and a field case for the theory. Landscape Ecology 24: 1-13.